"First, you turn the time circuits on." —Doc Brown explaining to Marty how the DeLorean time machine works.

The time circuits of the DeLorean time machine were an integral part to its functionality. They were coupled with an input device and a display. The display was divided into three sections: DESTINATION TIME, PRESENT TIME, and LAST TIME DEPARTED, all annotated with Dymo labels.

Years were limited to four digits and there were no possible negative years that could be reached, i.e. years before 1 AD. This means the DeLorean could travel to any time from 12:00 am on January 1, AD 1 to 11:59 p.m. on December 31, AD 9999. It is unclear what would happen if one were to travel to the latter date since, after only one minute, it would be the year AD 10000 and the time display would therefore no longer be able to show the present time.

Emmett Brown demonstrated its capabilities to Marty McFly after its first test, giving two well-known but erroneous dates as examples, the signing of the Declaration of Independence, July 4, 1776, and the birth of Christ, December 25, 0000. (In practice, these events could only be viewed by driving to the location of the event with the help of gas stations and freight ships, and then traveling back in time, since the DeLorean did not yet travel in space at this point in its construction.) Then, Doc Brown gave a third example by setting them to the day that he invented time travel, November 5, 1955. Doc set a day, month, year and time option into the time circuits.

The setup was apparently user-friendly, considering that Marty and Biff Tannen were both able to insert destination times quickly and easily. In order to enter a date and time, one pressed digits on the keypad in the order that they would be displayed, and then press the activation button. For example, to set the time to November 5, 1955 at 9:00 a.m., one would press NOV|05|1955|09:00 and hit ENTER. In order to add time in the afternoon of a date, the operator would likely have keyed 1300, 1400 etc. to represent 1:00 p.m. and 2:00 p.m. respectively.

Suppose you wanted to set date and time to, let's say, September 1st 2014, 4:00pm. You would first type the month on the keypad (09) and then the date (01) and then year (2014) and then hour (16). The sixteen for 4pm in 24 hour time. Finally you write the minute (00) and press enter. So altogether it would be (090120141600) and then enter.

Time Circuis malfunctioning
The time circuits malfunctioning, with the Destination Time section showing the date to which Doc will be accidentally sent to.

After the trip to 2015, the time circuits began malfunctioning, displaying "Jan 01, 1885", at midnight, in the destination slot in mid flight. A bolt of lightning triggered the malfunction to send the DeLorean from 1955 to 1885. Though the vehicle was in mid-air, the spin created by the lightning bolt allowed it to reach 88mph.
Note: The following section is considered non-canon or is disputed in canonicity.
When Marty came across a temporal duplicate of the DeLorean on May 14, 1986, the Last Time Departed display was blank until the car was shot in 1931.

After the DeLorean fell from a billboard in 1986B, the time circuits ceased to function altogether. Though Citizen Brown would repair them the best he could, the time circuits would fail to take the Delorean to its exact destination. Following examples include:

Citizen Brown arriving to the Courthouse Square 2 minutes after his departure when he intended it to be exactly after he departs.
Marty and Citizen Brown arrive in October 12, 1931 rather than August 26, 1931.
Note: The following section is considered non-canon or is disputed in canonicity.
After the time machine was rebuilt by Doc in 1991, the time circuits were constructed to be voice-activated, as well as being coupled with a device that allowed spatial displacement. It appears that the circuits could now display year numbers larger than 10,000.

Non-canon or disputable information ends here.
Interestingly, on board the Jules Verne Train, Doc still built a display that only contained four digits for the year, thus creating the 10,000 year problem again.[1].

"This is what makes time travel possible."—Doc Brown

The flux capacitor was the core component of Dr. Emmett Brown's time traveling DeLorean time machine and the following Jules Verne Train. Doc stated that the flux capacitor "is what makes time travel possible."

On November 5, 1955, Emmett Brown came up with the idea of the flux capacitor after slipping and bumping his head while standing on his toilet to hang a clock. The idea came to him in a vision he had after being knocked out. He drew up a schematic diagram of an inverted Y-shape with wires and stated "flux compression". He also performed some mild calculations on the paper.

Note: The following section is considered non-canon or is disputed in canonicity.

Since coming up with the idea, Doc spent many years and most of his family fortune trying to get it to work. By 1962, he had invented the temporal field capacitor. It was a prototype time machine that was able to send objects through time, but only during the lifespan of the device. It could only send objects forward through time, as sending objects even a few minutes into the past would cause a buildup of flux energy. If an object was sent any further back than that, the arrival of the object would overheat the capacitor and start an electrical fire. The problem with the build-up of flux energy was solved by the flux capacitor, allowing time travel to the future, as well as the past.

Non-canon or disputable information ends here.

The flux capacitor consisted of a box with three small, flashing incandescent lamps arranged as a "Y", located above and behind the passenger seat of the time machine. As the car neared 88 miles per hour, the light of the flux capacitor pulsed faster until it had a steady stream of light, which one was not supposed to look at as indicated by the Dymo warning label SHIELD EYES FROM LIGHT placed across the glass panel. The stainless steel body of the DeLorean also had a beneficial effect on the "flux dispersal" as the capacitor activated, although Doc was interrupted by the arrival of the Libyans before he could finish explaining it to Marty McFly fully. Accessing the flux capacitor safely required disconnection of the capacitor drive, as the Dymo warning label at the top of the unit — DISCONNECT CAPACITOR DRIVE BEFORE OPENING — pointed out.

The coils that can be seen across the front and along the rear sides can be referenced as the temporal demodulation coils (as used in the original blueprints of the vehicle). These play a key part to open a hole in the time barrier.

In order to travel through time, the vehicle integrated with the flux capacitor needed to be traveling at 88 mph (140.8 km/h), and required 1.21 gigawatts of power (1,210,000,000 watts), originally supplied by a plutonium-powered nuclear reactor. However, for the time machine's return trip (from 1955 back to 1985), plutonium was not available, so a lightning rod was connected directly into the flux capacitor and was used while the vehicle sustained 88 m.p.h. Plutonium was used once again for a trip forward in time at least 30 years, and at some point thereafter the plutonium reactor was replaced by a Mr. Fusion home energy generator from the future that was fueled by extracting hydrogen atoms from garbage.

The DeLorean once again came back to 1985 and proceeded to travel to 2015, where it was stolen by Biff Tannen, taken back to 1955 and returned to 2015 without Doc's knowledge. When they returned in 1985, they found it was a different present, so they traveled back to 1955 to fix the space-time continuum. The DeLorean was again struck by lightning in the year 1955, this time by accident. The lightning created an overload and caused a malfunction in the time circuits sending the vehicle back to January 1, 1885. Earlier, the 1885 date was already displayed before the lightning hit, after the LED read-outs flashed.

The DeLorean was then hidden in the Delgado Mine by Doc for seventy years because suitable replacement parts were not invented until 1947. [2]. It was recovered from the mine in 1955 and repaired by Doc's 1955 counterpart, thus restoring it to working order. Since both gasoline and garbage were available, the next trip back to 1885 was performed under the car's own power.

Due to a broken fuel line, the DeLorean's final trip from 1885 to 1985 was partially powered by a steam locomotive pushing the vehicle up to 88 mph while using Mr. Fusion to generate the 1.21 gigawatts required to activate the flux capacitor and break the time barrier. Doc returns to 1985 in a time machine made from a locomotive. This used a flux capacitor that Doc was able to power using steam (at least with the propulsion requirement for time travel), located on the front of the train, in place of the lamp.